The invention relates to a method for inspecting an object such as a wafer, and more particularly for inspecting an object comprising structures such as bumps or micro-bumps.
The field of the invention is, but not limited to, 2D-3D inspection and metrology for semiconductor industry.
Tridimensional structures such as bumps, micro-bumps, solder bumps, copper pilar, copper nails, Re-Distribution Layers (RDL), Under Bump metallization (UBM), metal patterns are more and more widely used for interconnections and other application in the semiconductor industry. With the evolution of the IC packaging, the critical dimensions of these structures tend to decrease and their density on wafers or chips tend to increase. In the same time there is a need for inspection systems capable of inspecting all of these structures at high speed during production, as a few defects are sufficient to render corresponding ICs defective.
Chromatic confocal technique is a well-known technique for tridimensional (3D) surface mapping or profilometry, in particular for semiconductor applications.
The technique relies on the use of a chromatic lens with an enhanced chromatism, whose focal length depends strongly on the optical wavelength. Each wavelength of the light crossing such lens is focused at a different distance, or in a different focal plane.
The chromatic lens is embedded in a confocal set-up with source and detection pinholes (usually made by optical fibers core) placed at the confocal planes of the chromatic lens to reject out-of-focus lens. When a reflecting surface is placed in front of the chromatic lens, only the light with the wavelength whose focal plane corresponds to the position of the surface is transmitted by the detection pinhole.
Detection is made by a spectrometer, which comprises usually a dispersing element and a linear or matrix sensor (CCD or CMOS) to acquire the intensity spectrum of the light. The height (or distance) of the surface relative to the chromatic lens is obtained by analyzing the intensity spectrum of the detected light.
Such set-up allows measuring distances on a single point at the time. So inspecting a full wafer surface by scanning all the surface may be very time-consuming. Actually, the factor limiting the measurement speed is the readout time of the linear sensor for acquiring the intensity spectrum.
Acquisition speed can be improved by providing several measurement channels in parallel.
We know for instance the document US 2015/0260504 which discloses an implementation of a confocal chromatic device in which several measurement channels are provided through a chromatic lens with several optical fibers. The sensor allows measuring distance or height at several points on the surface of the object simultaneously.
However, even if the acquisition rate is improved, the time for inspecting a full wafer surface remains very long.
Another issue when measuring or inspecting patterned wafers is the accurate localization of the measurement points relative to the existing structures. That issue is usually solved by using a 2D (bidimensional) inspection or imaging system such as a camera.
We know for instance the document U.S. Pat. No. 6,934,019 which describes an inspection system based on a chromatic confocal sensor which comprises also an imaging camera. The measurements require two steps: first acquiring an image of the wafer with the camera and computing a map of the locations of the structures to be measured; and second performing the height measurements.
However, the switching between the camera and the chromatic sensor is time consuming and the need for mechanical displacements to position either the camera or the chromatic confocal sensor above the structures to be measured may impact the positioning accuracy for the height measurement.
It is an object of the invention to provide a method allowing fast and accurate 2D (bidimensional or in-plane imaging) inspection of an object such as a wafer.
It is also an object of the invention to provide a method allowing fast and accurate 3D (tridimensional height measurements) inspection of an object such as a wafer.
It is also an object of the invention to provide a method allowing providing simultaneously or at least during a same scan and with minimized positioning uncertainty an intensity image (2D) and accurate height measurements (3D) on an object such as a wafer with patterns or structures.
It is also an object of the invention to provide a method allowing accurate locating of height measurement positions, and/or accurate positioning of height measurement probes relative to structures or patterns on an object such as a wafer.
It is also an object of the invention to provide a method allowing characterizing or inspecting structures (in 2D and/or in 3D) of an object such as a wafer in position and shape.
It is also an object of the invention to provide a method for inspecting bumps, trenches and other patterned structures on an object such as a wafer.